Such a known band of metal or plastics, as described in Austrian Pat. No. 292,391, has a U-shaped cross-section and at its edges numerous cutouts provided by stamping as well as tabs which are stamped out from the center of the band and bent round at right angles with holes through which two tensioning elements are guided in the form of cables. As soon as these elements are tensioned by a stretcher, bending of the band occurs evenly over the length of the band or irregularly as a result of the tensioning elements left hanging at the hole edges.
In addition, the wear of the cable-like tensioning elements in the regions of the holes damages these elements so that they must be replaced.